A series of experiments will be carried out to explore the mechanism of periodic enzyme synthesis during the cell cycle of eukaryotes. Saccharomyces cerevisiae will be used for these studies because it is one of the simplest eukaryotes, and it is easily synchronized and amenable to both genetic and biochemical analysis. Synthesis of the enzyme maltase will be examined because this enzyme is synthesized at different periods of the cell cycle in different strains, and specific genes (the MAL genes) are thought to be responsible for the differences in the timing of periodic synthesis. 1. A series of strains has been constructed carrying different MAL genes in a common background. These strains will be used a) to confirm the role of the MAL genes in the regulation of maltase synthesis during the cell cycle, b) to determine the differences in the MAL gene products responsible for maintaining different enzyme levels during asynchronous growth. 2) Existing regulatory mutations which affect the ability of maltase to be induced by maltose or repressed by glucose will be examined for their effects on the synthesis of maltase during synchronous and asynchronous growth. 3) New regulatory mutants will be isolated using a direct selection procedure. 4) Maltase structural gene mutants will be looked for using a positive selection procedure.